"Name forms" with slapo-constraint

HI!
I tried to use slapo-constraint on entryDN when adding new entries.
Kind of poor man's name forms...
constraint_attribute
entryDN
regex mail=[^,],departmentNumber=[0-9]+,O=MyOrg
restrict="ldap:///O=MyOrg??sub?(objectClass=inetOrgPerson)"
But looking at the trace log it seems this is not evaluated at all probably
entryDN is not present in the added entry and not yet generated.
Any hints whether that might work?
Ciao, Michael.